This application claims the benefit of Japanese Patent application No. 2001-065242 which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a zoom lens system suitable for a video camera and a digital still camera using a solid-state imaging device and, in particular, relates to a compact zoom lens system having a zoom ratio of 3.5 or more, an angle of view in a wide-angle end state of about 60xc2x0, and superb optical performance.
2. Related Background Art
So far zoom lens systems disclosed in Japanese Patent Application Laid-Open No. 11-52246 have been known to be those suitable for a solid-state imaging device.
However, zoom lens systems disclosed in Japanese Patent Application Laid-Open No. 11-52246 have relatively small zoom ratio of about three. Therefore, it has been a problem to be insufficient zoom ratio.
The present invention is made in view of the aforementioned problem and has an object to provide a compact zoom lens system, which is suitable for a video camera and a digital still camera using a solid-state imaging device, having a zoom ratio of 3.5 or more, an angle of view in a wide-angle end state of about 60xc2x0, and superb optical performance.
According to one aspect of the present invention, a zoom lens system includes, in order from an object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power. The front principal point of the second lens group is located to the object side of the most object side surface of the second lens group. The distance between the first lens group and the second lens group is decreased and the distance between the second lens group and the third lens group is increased, when the state of lens group positions is changed from a wide-angle end state to a telephoto end state. The following conditional expressions (1), (2), and (3) are satisfied:
0 less than H1/fw less than 0.6xe2x80x83xe2x80x83(1)
1.0 less than H2/fw less than 2.2xe2x80x83xe2x80x83(2)
1.6 less than f3/f2 less than 3.0xe2x80x83xe2x80x83(3)
where H1 denotes the distance between the front principal point of the second lens group and the most object side surface of the second lens group, H2 denotes the distance between the rear principal point of the second lens group and the most image side surface of the second lens group, fw denotes the focal length of the zoom lens system in the wide-angle end state, f2 denotes the focal length of the second lens group, and f3 denotes the focal length of the third lens group.
In one preferred embodiment of the present invention, the first lens group and the second lens group are moved and the third lens group is fixed when the state of lens group positions is changed from the wide-angle end state to the telephoto end state. The following conditional expression (4) is satisfied:
2.6 less than M/fw less than 3.0xe2x80x83xe2x80x83(4)
where M denotes the moving distance of the second lens group along the optical axis between the position locating the most object side and that locating the most image side upon zooming.
In one preferred embodiment of the present invention, the second lens group is composed of, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having negative refractive power, and a fourth lens unit having positive refractive power.
The following conditional expressions (5), (6), (7), and (8) are satisfied:
0.6 less than f21/f2 less than 1.0xe2x80x83xe2x80x83(5)
0.7 less than |f22|/f2 less than 2.0(f22 less than 0)xe2x80x83xe2x80x83(6)
0.2 less than |f23|/f2 less than 10.0(f23 less than 0)xe2x80x83xe2x80x83(7)
1.2 less than f24/f2 less than 3.0xe2x80x83xe2x80x83(8)
where f21 denotes the focal length of the first lens unit, f22 denotes the focal length of the second lens unit, f23 denotes the focal length of the third lens unit, and f24 denotes the focal length of the fourth lens unit.
In one preferred embodiment of the present invention, the first lens unit includes a cemented lens constructed by, in order from the object side, a double convex positive lens having an aspherical surface facing to the object side cemented with a negative meniscus lens.
In one preferred embodiment of the present invention, the second lens unit includes a cemented lens constructed by, in order from the object side, a double convex positive lens cemented with a double concave negative lens.
In one preferred embodiment of the present invention, the third lens unit includes a negative meniscus lens having a convex surface facing to the object side.
In one preferred embodiment of the present invention, the fourth lens unit includes a positive meniscus lens having a convex surface facing to the object side.
In one preferred embodiment of the present invention, the first lens group is composed of, in order from the object side, a negative meniscus lens having a convex surface facing to the object side, a double concave negative lens having a stronger concave surface facing to the image side, and a positive meniscus lens having a convex surface facing to the object side.
In one preferred embodiment of the present invention, the third lens group is composed of a double convex positive lens having at least one aspherical surface.